The invention relates to the field of solar collector systems and more particularly to a closed loop solar collector system having a linear parabolic reflector and a linear receiver tube placed at the focal line of the reflector for vaporizing a quantity of heat transfer fluid contained in the receiving tube.
Solar heating holds much promise for supplying a portion of the energy needs of the nation in the near future. Unlike many forms of energy, solar energy is non-polluting, and depletes none of our valuable hydrocarbon natural resources. Solar energy is thus essentially free and can be utilized directly by individuals, unlike conventional electric power which must be generated in a central location to be cost effective.
While the source of solar energy, the sun, is free, there are several limitations on the state of the art of solar collectors which affects their cost effectiveness versus more conventional energy sources, such as coal, oil or nuclear. The effectiveness of a solar energy collector depends primarily on its siting, e.g., locations which have a high number of substantially cloudless days, such as the south-western United States, are good sites for solar collectors, whereas areas such as the Pacific northwest, which have few cloudless days are less desirable. Another factor is the type of collector used. Presently, both flat collectors and concentrating collectors are known. Recent studies have implied that concentrating collectors are equal to, or better than the flat plate collectors in terms of efficiency under both cloudless and partly overcast (diffuse) lighting conditions. While flat plate collectors are somewhat simpler to construct than those of the concentrating type, the higher temperatures (over 100.degree. C.) achievable by concentrating collectors, result in higher theoretical efficiencies than the flat-plate type.
Concentrating collectors come in many types, as outlined in a Sandia Laboratory Report No. 78-0949 entitled "Linear Concentrating Solar Collectors-Current Technology and Applications" (1978). From a review of the art, it is apparent that in most cases the simpler the design of the concentrating collector and allied systems, the higher the efficiency of heat conversion is. In addition, a simple system usually is more reliable and more cost effective from a manufacturing standpoint. Systems which must use an outside source of electrical power to operate auxiliary pumps or tracking mechanisms preclude their use at remote or wilderness sites. Complicated tracking devices (for following the diurnal or annual motion of the sun) or exotic materials merely place solar energy one step further from utilization by the ordinary citizen.
In a typical linear concentrating collector system, a trough-like linear parabolic reflector is mounted either east-west or north-south, depending on the type of sun tracking system to be used. A receiver tube, constructed from glass or metal and overcoated with a heat absorptive coating, is disposed along the focal line of the linear parabolic reflector. A vaporizable heat transfer fluid, such as water, Freon, or Therminol-66 (a heat transfer oil) is contained within the receiver tube. Solar energy concentrated upon the receiver tube by the parabolic reflector heats the fluid and causes it to vaporize. The fluid absorbs a great deal of heat energy when vaporized. Depending upon the exact design of the collector and the type of transfer fluid used, peak efficiencies of approximately 60% with receiver tube output temperatures (of the vaporized heat transfer fluid) of from 100.degree. C. (for water) to 315.degree. C. (for Therminol-66) have been achieved by present day systems.
While the general form of such concentrating collector systems is well-known, many areas of collector system designs have yet to be fully explored. For example, a concentrating collector can be operated as an open loop or a closed loop system. In an open loop system, the heat transfer fluid, generally water, flows once through the receiver tube, is vaporized into steam, and then the steam is used to drive a compressor, turbine or other such device. The spent, low enthalpy steam is then allowed to escape into the atmosphere. While this system offers the benefit of a high theoretical operating efficiency because of the large difference between the input temperature of the fluid and the output temperature of the steam, such a system requires a continuous source for replenishing the heat transfer fluid and is wasteful of the fluid.
A closed loop system, on the other hand, while having a slightly lower theoretical operating efficiency than the open loop system, has the advantage that no outside source of heat transfer fluid is needed since this system is self-contained. In a closed loop linear concentrating solar collector system, the vaporized fluid from the receiver tube flows into a heat exchanger, where the heat from the vaporizer fluid is extracted and the fluid is condensed. The condensed fluid then flows, or is pumped, to a storage tank for re-use.
One problem with closed loop linear concentrating solar collector systems is that a certain amount of heat transfer fluid must be precisely metered into the receiver tube during each vaporization cycle. If too much fluid is released into the receiver tube, vaporization of a useful quantity of the fluid is slow, thus reducing the operating efficiency of the system. If too little fluid is released into the receiver tube, or if the tube is allowed to run dry, the tube itself can be damaged due to overheating. In addition, such a fluid metering device should be capable of operating continuously and accurately for long periods of time in various weather conditions. Such a device should operate efficiently at various angles, since the orientation of the solar collector changes day by day to track the annual motion of the sun. Further, a linear concentrating solar collector system should be simple to construct, reliable in operation and preferably require no outside source of power for operation.
It is therefore an object of the present invention to provide a closed loop linear concentrating solar collecting system having improved means for regulating the flow of a heat transfer fluid through the system.
It is another object to provide a closed loop concentrating solar collector system which requires no source of power, other than the sun, for operation.
It is a further object to provide a closed loop linear concentrating solar collector system including one or more modular heat exchangers for extracting useful heat from the system.
It is yet another object to provide a closed loop linear concentrating solar collector system which is economical to construct and reliable in operation, and which has a relatively high thermal operating efficiency.